The chief aim is to provide a detailed, well-illustrated account of the developing human nervous system based on histological examination of staged embryos (using the internationally accepted Carnegie staging system of Streeter and O'Rahilly). In addition to polychrome histological stains, silver impregnation of nerve fibers will be employed. Another major feature will be the preparation of precise graphic reconstructions, including the use of the Perspektomat, which is a special, new drawing instrument for parallel perspective views. Appropriate data are to be incorporated into information on the Carnegie Collection already in computer-readable form at The Johns Hopkins Medical Institutions and analysed there. The sequence and timing of developmental events can thereby be determined with a precision not available heretofore. The degree of variation found at each stage will be assessed, which has not been done previously. Various tracts of the developing brain so far studied in inadequate detail will be mapped. Preliminary studies in this laboratory indicate that the brain is more highly developed at early stages than has been recorded. Congenital anomalies encountered (several have been already) will be analysed in detail and reconstructions made where appropriate. The main long-term objective is to contribute to a precise and detailed (in the sense of Muller and O'Rahilly, Amer. J. Anat., 1980, 159:33-58) study of the embryonic human nervous system at each developmental stage. (Animal studies are important but are not a substitute for the direct investigation of the human.) The cief scientific disciplines involved are human embryology and pathology, supplemented by computerized sorts and tabulations. The development of the nervous system is of fundamental biological and medical importance, and ultimately the correlation of such studies with the onset and development of activity in the human could be significant in elucidating the development of intelligence and behavior. Moreover, the study of normal and abnormal development leads to a better understanding of congenital anomalies. Those of the central nervous system, which are of great clinical importance, constitute about 10% of the total "load" of malformations.